Diesel fuel is subject to many standard specifications, including cold flow properties, such as Cloud Point and Cold Filter Plugging Point (CFPP). These cold flow properties are important to insure that the fuel reliably flows to the engine in low ambient temperature. In the US, ASTM D975 shows the 10% minimum ambient temperature per state as a guide to acceptable diesel cold flow properties. In the 48 contiguous states, the lowest minimum ambient temperature value is −34° C. Additionally, the lowest value reported for Alaska is −45° C. Similarly, home heating oil is subject to cold flow property specifications such as Pour Point.
Biodiesel is gaining growing acceptance as a diesel fuel component. ‘Biodiesel’ is typically fatty acid esters made from vegetable oil triglycerides, which can include various crops or waste oil, or other animal fats. The raw vegetable oil or animal fat triglycerides are reacted with methanol to form fatty acid esters, which have a viscosity within the diesel specification. Processing of algae can also yield similar triglycerides. The common term is “FAME,” which stands for fatty acid methyl ester. A separate ASTM specification has issued that covers Biodiesel (D6751-07) when blended with conventional diesel, but some of the specifications are not consistent with conventional diesel specifications required for the mixed blend. For example, the biodiesel Cloud Point spec is shown as ‘report only’, with a footnote that it is usually higher than conventional diesel fuel and this need to be taken into consideration. Biodiesel fuels often have high cloud points. As a result, blends of biodiesel and conventional diesel may render the total blend unsuitable in terms of cold flow properties.
EP 1741767 A1 and 1741768 A1 describe methods for hydroprocessing diesel range feeds based on biocomponent sources, such as vegetable or animal fats/oils. The hydroprocessing methods include exposing the biocomponent feed to hydrotreating conditions, followed by a hydroprocessing step for isomerizing the feed. Isomerization catalysts identified in EP 1741767 A1 and EP 1741768 A1 include SAPO-11, SAPO-41, ZSM-22, ZSM-23, and ferrierite. The isomerization catalysts are described as also including a Group VIII metal such as Pt and a binder such as alumina. The lowest cloud points identified in the references are between −14 and −22° C. The levels of n-paraffins remaining in the isomerized diesel products were not specified.
US Published Patent Application 2007/0006523 describes methods for producing diesel fuels from a Tall Oil Fatty Acid (TOFA) fraction. The TOFA fraction is described as including triglycerides present in biocomponent feeds, such as rapeseed oil, sunflower oil, or palm oil. The methods include hydrotreatment followed by isomerization. The most suitable isomerization catalysts are described as catalysts with low acidity. SAPO-11 bound with alumina and ZSM-22 or ZSM-23 bound with alumina are provided as examples of isomerization catalysts. The isomerization catalyst is also described as including a supported Group VIII metal such as Pt. No cloud points are provided for the diesel fuel products. The lowest reported number for the amount of n-paraffins in an isomerized product is 13%.
US Published Patent Application 2006/0207166 describes methods for hydroprocessing biocomponent feeds in a single step. The single step performs both hydrodeoxygenation and hydroisomerization. The catalyst for the single step is described as including both a metal component and an acidic component. The metal component is described as platinum or palladium. A wide variety of zeolites are described for the acidic component. A porous solid support may also be present. The lowest cloud points reported for diesel fuels made according to the process described in US 2006/0207166 are between −11 and −16° C. A cloud point below −20° C. is also reported in a comparative example. After processing, the reported diesel products had n-paraffin contents of at least 14.5%.
What is needed is a method for producing biocomponent based diesel fuels with improved properties to facilitate use in the commercial fuel supply. Preferably, the method would allow for production of diesel fuels that satisfy any current cold flow property requirements while also providing improved cetane.